How could the Walpolean oligarchy be at once exclusive and stable?

How could the Walpolean oligarchy be at once exclusive and stable? The force and longevity of the Whig dominance of politics in Britain at this time, culminating in Walpole's record term of office, created the most clearly defined period of oligarchical rule in British history. The 'Whig oligarchy' created a powerful and potentially dangerous division in the country, as the Tories were kept from office year after year. With such exclusion, there was real danger …

…the understandings and constitutional conventions developed during the 1690s. Yet such successes were only central in ensuring the long term survival of the Whigs as the governing party, and were somewhat incidental to the broader stability of the nation. With the political nation agreed that stability was of prime importance, it was possible for an oligarchic and exclusive government to exist, whilst the considerable successes of that government went on to ensure its prolonged existence.